Many pet parents don't mind sharing a kiss with their dog; they see it as an indication that they care for each other. However, there's the possibility that such demonstrations of affection may not be good for your health. Dental disease and bacterial infections can be shared between dog and pet parent, so it's not surprising that health professionals would prefer you gave your dog a hug or belly rub instead.
Dogs use their nose and mouth to investigate their environment. They eat things they shouldn't and lick things that we really wouldn't find very pleasant. They've been known to eat poop, dead birds, and rotten leftover food. They also use their tongue to lick their body, including some parts that aren't likely to be very hygienic. Having that mouth and tongue close to your face could spread bacteria that you'd rather not have.
Dogs can also carry roundworm and parasites such as giardia, roundworm, and hookworm, which can potentially spread to people. The opinion of microbiologists is divided – some believe that the chances of parasites being transmitted to humans is very low, while others feel the risk is there and kissing your dog isn't a good idea at all.
Japanese researchers evaluated the number and type of "periodontopathic" bacteria in both dogs and people. These bacteria invade the tissues surrounding the teeth and contribute to gum disease. While the different species of bacteria occur with varying frequency in dogs and people, four species were found in both dogs and their pet parents who were in frequent close contact with their canine companions. This suggests that it's possible for bacteria to be spread between pet parents and dogs, and contribute to gum disease in both species.
Apart from the health risks to you, kissing your dog could be more stressful for him than you think. Not all dogs appreciate a face pressed close to them but it isn't always obvious to a pet parent that they feel this way. Watch for signs such as leaning away from you, turning their head away, and licking their lips. Some dogs may go as far as snapping if they're really distressed by an attempted kiss. If your pup doesn't appreciate being smooched, then look for another way of showing you care such as a massage or a good grooming session.
The risks of gum disease, parasites, and infections are concerning but there are degrees of risk. Those with weaker immune systems are more likely to become unwell from kissing and being kissed by a dog. This group of people includes babies, the elderly, and those being treated for cancer.
The best tactic for preventing illness is to avoid kissing your dog completely or allowing them to kiss you. However, most pet parents will do it at some time in spite of the risks. Regular veterinary care, treating your pet for worms, and preventing them from scavenging can all reduce the chances of them sharing infections or parasites with you.